In the production of packaging materials of the type employed for liquid packages of the single-use disposable type, the common point of departure is a core layer of paper or paperboard to which are laminated different layers of thermoplastics, and possibly aluminium foil. Hitherto, the standard procedure has been that the ready-laminated packaging material is printed on its outside, to obtain an external artwork decor. The finished packaging material, most often consisting of a material web, is reformed in a filling machine, sealed along a longitudinal joint seam, filled with the contemplated contents, transversely sealed and severed into individual packaging containers which are possibly finally formed into finished packaging containers. The finished packaging containers are often provided with some form of opening arrangement.
Recent times have also seen the introduction of printing direct onto the film which is to constitute the outer layer of the packaging material, and the ready-printed film is thereafter laminated onto the core layer. By such means, it is possible to obtain an improved print quality, at the same time as reducing material losses. This is because the printing is that stage of the production process for packaging material which causes the greatest material loss or wastage. When printing is carried out on the film, only the film thus constitutes waste material or spillage, while it previously consisted of an entire finished packaging material with core layer and all laminate layers.
In the production of the finished packaging material, common practice has also been to provide, in the process, some form of opening arrangement such as perforations, holes for drinking straws, and the like. These arrangements were previously made in the core layer and those places which were employed for the different outer and inner layers were easy to tear off or penetrate.
In order to print direct on the film, which is then laminated onto the core layer, use is made of oriented plastic films, such as oriented polypropylene (OPP-film). These films have a high modulus of elasticity and, when they are employed for the outer layer of the packaging material, they must be pre-treated, i.e. be perforated on sections of the surface before lamination to the core layer takes place. This is because these films display such toughness that they are difficult to tear off or penetrate without a preparatory perforation. They are perforated across a surface which corresponds to or is larger than the tear indication or drinking straw hole which is provided in the core layer.
In order to obtain a perforation surface in a thin plastic film, use has been made of solid tools which are machined from one homogeneous piece and which are surface treated, for example by spark erosion so as to obtain a number of barbs or teeth upstanding from the surface, whose extent corresponds to the desired perforation pattern.
Such tools are intended to be mounted in a roller, two to four tools about the circumference of the roller, depending upon the register length and the size of the roller. The thin plastic film is caused to pass between the roller provided with the tools and a counter roller which is preferably provided with a rubber surface.
These solid tools are expensive to manufacture and they require meticulous and accurate setting on mounting in the roller. It has also proved difficult to manufacture a tool with teeth or barbs so densely packed for the formation of the desirable perforation pattern.